Hi, I wanted to use a boost::container::small_vector for 64-bit aligned doubles (to be used with SSE): But it seems that the "small-buffer" has a fixed calculated alignment which only depends on the value-type and does not consider the alignment of the (dynamic) allocator. Is this intended or is it a bug in the implementation? best regards Tobias -- View this message in context: http://boost.2283326.n4.nabble.com/small-vector-alignment-tp4692694.html Sent from the Boost - Dev mailing list archive at Nabble.com.
I wanted to use a boost::container::small_vector for 64-bit aligned doubles (to be used with SSE):
typedef boost::container::small_vector<double, 16, aligned_allocator<double, sizeof(__m128)> > sse_double_vector;
But it seems that the "small-buffer" has a fixed calculated alignment which only depends on the value-type and does not consider the alignment of the (dynamic) allocator.
Hi Tobias, Yes, it looks like small_vector does not expose any mechanism for you to control the alignment of its internal "small" storage. So you can only expect that for 'double' the storage is at least alignof(double) aligned. (The aligned_allocator will only apply after that 'small' storage is exhausted of course.) One solution could be to use std::vector instead with aligned_allocator_adaptor with in conjunction with another custom allocator for a kind of small-size optimization e.g. sso_storage<N> s; std::vector<double, aligned_allocator_adaptor<sso_allocator<double, N>, alignof(__m128)> > sse_double_vector(sso_allocator<double, N>(s)); (i.e. The 'sso_allocator' and 'sso_storage' is what you would have to implement.) I would suggest also opening an feature request to Ion to allow providing a mechanism to specify the alignment of the small_vector storage type. Glen -- View this message in context: http://boost.2283326.n4.nabble.com/small-vector-alignment-tp4692694p4692701.... Sent from the Boost - Dev mailing list archive at Nabble.com.
On 21 March 2017 at 07:55, Glen Fernandes via Boost <boost@lists.boost.org> wrote:
I would suggest also opening an feature request to Ion to allow providing a mechanism to specify the alignment of the small_vector storage type.
This (the mechanisme) is very usefull (and I can stop to r-m-o due to this issue). +1 degski
Hi Glen, thanks for the tip with the allocator. In the meantime, I came up with another solution: I made a new double type (not just a typedef) and specialized its allignment: Its only drawback is that accessing the "data()" member returns a double_m128* which has to be reinterpreted as a double*. Tobias -- View this message in context: http://boost.2283326.n4.nabble.com/small-vector-alignment-tp4692694p4692891.... Sent from the Boost - Dev mailing list archive at Nabble.com.
I've made a pull-request <https://github.com/boostorg/container/pull/47> for the container-module. Comments are wellcome. Tobias -- View this message in context: http://boost.2283326.n4.nabble.com/small-vector-alignment-tp4692694p4693060.... Sent from the Boost - Dev mailing list archive at Nabble.com.
On 27/03/2017 12:42, Tobias Loew via Boost wrote:
I've made a pull-request <https://github.com/boostorg/container/pull/47> for the container-module. Comments are wellcome.
Tobias
The alignment could be part of the allocator's compile-time properties. At least, this does not require passing the alignment as an extra template parameter. Best, Ion
participants (4)
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degski
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Glen Fernandes
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Ion Gaztañaga
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Tobias Loew